In the midst of increasing momentum to suppress carbon dioxide emissions and depletion of petroleum resources, expectations have been placed on the development of compact vehicles employing only electric power (for example, lead-acid batteries or other secondary batteries) as motive power.
Of these, lead-acid batteries stand up to harsh usage conditions and have moderate mass, and so are useful as the motive power source of, for example, hauling vehicles.
Various methods for efficient charging of lead-acid batteries are being studied. In Patent Document 1, a method is considered of repeatedly charging the next time with a charging quantity (synonymous with charged electrical quantity) slightly greater than the discharge quantity (synonymous with discharged electrical quantity) discharged after the previous charging. It is stated that by means of this method, except for prescribed refresh overcharging (putting the lead-acid battery into an overcharged state, and causing recovery of the capacity of the lead-acid battery), while avoiding overcharging, declines in the capacity of the lead-acid battery due to undercharging can be prevented.
In a lead-acid battery, until the number of charge/discharge cycles, consisting of a charge cycle and an immediately following discharge cycle, reaches a prescribed number of cycles (or, until the accumulated value of the discharge electrical quantity reaches a prescribed value), by charging the next time with a charging quantity slightly greater than the discharge quantity discharged after the previous charging, a decline in capacity due to undercharging can be prevented. However, it has been found that if the number of charge/discharge cycles exceeds a prescribed number of cycles (or, if the accumulated value of the discharge electrical quantity exceeds a prescribed value), by charging the lead-acid battery with a charging quantity slightly greater than the discharge quantity discharged after the previous charging, the lead-acid battery enters an overcharged state, and degradation occurs.    Patent Document 1: Japanese Patent Application Laid-open No. 2003-219571